This invention relates to traction winches.
Some present day winch systems for controlling tension on a mooring line employ a pair of parallel traction drums and a storage drum, with the rope coming from the load passing several times around the pair of traction drums and then to the storage drum. The traction drums hold the rope by friction and operate as the principal power means for drawing in or braking means for paying out line whereas the storage drum upon which the low tension end of the line is spooled, supplies the tension required to maintain the frictional forces between the rope and the traction drums. For synthetic rope to be capable of handling the high tension involved in mooring and towing applications, the rope diameter must be quite large and may in some case be as large as or greater than 20 inches in circumference. Spooling such large diameter rope on a conventional storage drum is unwieldy. Moreover, conventional storage drums are with difficulty designed to withstand the extremely high compressive forces which would be imparted thereto by such rope wound thereon even under the moderate back tension. Also, if the rope lengths are long, usually the case, the storage drum has to be inconveniently large.
In addition, in present day winch systems of the type described above, torque is unevenly distributed between the two traction drums imposing large and inefficient loads on the drive mechanism. In some instances the two traction drums may even work against each other.
A further problem with existing traction winches is that the rope elastically contracts as its tension diminishes in passing through the winch. The changing rope length, which is especially pronounced in synthetic fiber ropes, must be accommodated by sliding against the drums, with resulting rope wear.
Conventional systems using semi-circular grooves must make many turns around the traction drums to develop adequate friction. This causes very high bearing loads on the drum shafts. This problem can be partly alleviated by making successive grooves for the rope turns smaller in diameter, but a different ratio of variation is required for each tension or rope material prohibiting general use of the equipment.